1. Field of the Invention
The present invention relates generally to the production of electrical power using geothermal fluids and, more particularly, to a method for the production of useable steam and non-toxic solids from geothermal brine. The steam may be used for the production of electrical power through steam turbines, or the like, and the non-toxic solids are suitable for beneficial use in an environmentally acceptable manner.
2. Discussion of the Prior Art
Generally speaking, heat from the earth is an inexhaustible resource. The great majority of this heat is unavailable except in those instances where the hot magma from the earth's interior has come in contact with water, which often produces surface manifestations, such as hot springs and geysers. Geothermal brine, at temperatures of over about 500.degree. F., may be withdrawn from large subterranean aquifers which have been found in many areas of the world. Unfortunately, such brine is not inherently pollution-free. In fact, the utilization of such brine for power generation may produce adverse environmental impacts in view of the present environmental standards.
Brine and steam from naturally occurring geyser activity has given enjoyment to mankind since antiquity and, over the years, extraction wells drilled into the earth's surface to intercept the subterranean aquifers have produced a steady, dependable supply of hot pressurized brine to the earth's surface. Brine removed from the aquifers is flashed into useable steam leaving a spent brine having a super-saturated amount of solids therein, which subsequently precipitate to form a sludge.
Although the hereinabove-recited principal of extracting steam from geothermal brine is easily stated, its implementation is not without adverse environmental impact. In fact, the environmental considerations frequently prevent economical production of electrical power. Most of the environmental impact problems are associated with the composition of the geothermal brine. As hereinbefore noted, geothermal brines may have temperatures over about 500.degree. F., with pressures in the range of between about 400 to about 500 psig. At these temperatures and pressures, the geothermal brine easily leaches large quantities of salts, minerals and elements from the aquifer formation. As is well known, brine compositions vary from aquifer to aquifer, but typically contain high levels of dissolved silica, dissolved gases as well as dissolved toxic solids comprising, for example, antimony, arsenic, copper, lead and zinc, the toxic solids being most objectionable from an environmental impact point of view. As may be expected, flashing of the geothermal brine causes a super-saturated concentration of silica and dissolved toxic solids which can precipitate from the brine.
Without preventative measures, the impurities typically precipitate as a tough scale throughout the process equipment, including the reinjection wells for pumping the spent brine back into the aquifer for replenishment. Experience has shown that in high pressure, brine-flashing vessels, heavy metal sulfide and silicate scaling is the most severe problem, while in the comparatively low temperature atmospheric flashing portions of the system the scaling usually comprises silica and hydrated ferric oxide.
Disposal of spent brine is accomplished by injection, or pumping, of the spent brine down a nonproducing well into the aquifer. The motive for this procedure is to prevent contaminants in the brine from entering the environment and to return unusable enthalpy and mass to the reservoir, thus reducing thermal pollution and perhaps prolonging the reservoir lifetime. Unfortunately, the precipitated contaminants in the spent brine may eliminate or drastically reduce the permeability of the formation surrounding the injection well. Ultimately, this procedure might destroy the overall permeability of the aquifer. Hence, such precipitated solids must be removed from the spent brine before injection thereof in order to maintain the aquifer integrity and prevent blocking of the injection well itself, which may cause costly abandonment thereof or reconditioning in order to rejuvenate its functionality.
A considerable amount of effort has been directed toward developing effective processes for removing solids from the spent brine while at the same time eliminating or at least very substantially reducing silica scaling in the flashed geothermal brine handling systems. For example, U.S. Pat. No. 4,439,535 to Featherstone, et al, discloses the induced precipitation of scale-forming materials, principally silica, from the brine in the flashing stage by contacting the flashed brine with silica or silica-rich seed crystals. This procedure induces silica, precipitating from solution, to deposit onto the seed crystals rather than on equipment surfaces. Because the seed crystals provide a relatively large surface area for receiving precipitated silica compared to exposed surfaces of the flashing vessels and equipment, a majority of the precipitated silica is captured by the seed crystals and prevented from combining as a hard scale on interior equipment surfaces.
Unfortunately, seeding to capture precipitating silica as well as other solids from the flashed brine results in an increased amount of suspended solids which cannot be effectively disposed of by injection thereof. As hereinbefore noted, the suspended solids include heavy metal elements and compounds which may be toxic and must be treated as hazardous wastes if they are present in amounts greater than that defined by environmental standards.
In a typical geothermal brine power plant, suspended solids are settled from the spent brine in a clarifier. The clarifier separates or concentrates the precipitated solids into a sludge and a clarified brine overflow having a relatively small amount of suspended solids. The clarifier also produces the seed material useable for capturing precipitating solids as hereinbefore described. In this procedure a portion of the silica precipitate sludge is removed from the reactor clarifier and introduced into the flashed crystallization stage. The remainder of the sludge, in a typical facility, is dewatered and disposed as a solid waste.
The amount of waste can be considerable. For example, for a 10 megawatt power plant which requires a brine flow rate of about 1.2 million pounds per hour, more than 6 tons a day of sludge may be produced. Under heretofore known methods for operating a geothermal steam power plant, this sludge includes toxic elements and compounds which, as hereinbefore mentioned, are considered hazardous unless they appear in amounts lower than the standards set by government authorities. As should be appreciated, the costs associated with disposal of toxic sludge can be substantial and are expected to increase as toxic waste dumps become more scarce and/or remotely located.
The clarified brine is pumped into injection wells, but substantial amounts of clarified brine must be returned to the aquifer during operation of a geothermal power plant facility. For example, a 10 megawatt geothermal brine power plant, as hereinbefore mentioned, has a brine flow rate of 1.2 million pounds per hour. Consequently, even small amounts of fine suspended solids in the injected brine not removed by the clarifier can cause the injection wells to plug and thereafter become inoperable or inefficient in disposing of the geothermal power plant effluent. Significant costs are associated with reconditioning of a plugged injection well. For example, as much as a million dollars may be expended in order to recondition a well; hence, it is imperative to reduce the amount of suspended solids in the geothermal power plant effluent to as low a value as possible. When this is accomplished, however, more of these solids removed from the brine must be disposed of.
Typically, to separate fine particles from the spent brine, filters and ponds are used in combination with the clarifier. After the bulk of solid particles is separated and removed as a sludge from the clarifier, the brine is filtered through a set of filters, interconnected in series and/or parallel, which are designed to remove suspended particles from the brine.
Ponding of brine is also a common method of concentrating solids. In this procedure the brine containing fine suspended solids is pumped into large outdoor vats or ponds, and allowed to stand for a time sufficient to allow the fine solids to settle to the bottom of the pond. Thereafter the liquid is skimmed off and the accumulated sludge is dried and sent to toxic waste dumps along with dried sludge from the reactor-clarifier.
The present invention is directed to a method for the production of useable steam from geothermal brine while simultaneously producing a clarified liquid suitable for injection into the aquifer without significant plugging thereof, and solids having toxic elements below those limits defined as hazardous by government agencies such as the State of California. As can be appreciated from the hereinabove discussion with regard to the costs associated with the disposal of toxic wastes, the methods of the present invention provide significant economic benefit. The economic feasibility of recovering useful steam from geothermal brine sources hinges upon operating the plant in an environmentally acceptable manner without the occurrence of costs associated with the handling of toxic materials. These considerations become more important each day as public awareness over the environment hightens and stricter standards continue to be implemented by government agencies in order to regulate the production and disposal of waste considered toxic.
Additional advantages and features of the invention will become apparent to those skilled in the art from the following description when taken in conjunction with the accompanying drawings.